The present invention relates generally to a structure for deflecting a mirror of a laser cavity and, more particularly, to a path length control device having a driver with a deflectable membrane portion and a coefficient of thermal expansion which matches the coefficient of an associated mirror.
Ring laser gyros are typically equipped with one or more transducer-controlled mirrors to compensate for thermal expansion of the gyro body and maintain the path length constant over an operating temperature range. The range of mirror motion must be sufficiently broad to permit acquisition of a resonant mode of the gyro cavity and to compensate for expansion or contraction of the gyro block. In the case of a 28-centimeter gyro having a block of Zerodur or other suitable glass-ceramic material, two transducer-controlled mirrors must move up to 25 microinches (0.64 microns) to acquire an operating mode and up to an additional 110 microinches (2.79 microns) to compensate for expansion or contraction of the gyro block over a range of -40 degrees to +90 degrees Celsius. Thus, it is necessary to provide for at least 135 microinches (3.43 microns) of mirror movement in a typical 28-centimeter gyro.
Many transducer-controlled mirrors are of the membrane type, in which a central reflective surface is mounted to a deflectable glass-ceramic or quartz membrane for movement in a preselected axial direction. Piezoelectric transducer elements are used to deflect such mirrors in response to an electric signal derived in part from the output of the gyro.
Prior devices for "driving" membrane-type mirrors in a deflection mode have typically been composites of different materials, some of which have coefficients of thermal expansion much larger than the coefficients of the mirrors themselves. This can cause serious problems. For example, a common form of driver has a plurality of piezoelectric elements positioned directly behind and in contact with a central reflective portion of a mirror membrane to displace the reflective portion in response to an electrical input. Such devices are disclosed in U.S. Pat. Nos. 4,281,930; 4,410,274 and 4,410,276. In these devices, thermal expansion or contraction of the piezoelectric elements produces extraneous mirror movement for which the feedback system must compensate. This reduces the degree of compensation available to counteract expansion and contraction of the gyro block and reduces the temperature range over which the gyro can operate. The problem is aggravated by the fact that piezoelectric elements typically have coefficients of thermal expansion hundreds of times as great as the materials from which gyro mirrors and gyro blocks are made.
Another known device for deflecting a mirror of a ring laser gyro is illustrated in FIGS. 5A and 5B, wherein a pair of piezoelectric elements 10 and 12 are carried on opposite sides of a membrane 14 of a metallic driver body 16 to deflect the driver in response to an electrical input. The metallic driver body is mounted to the back side of a mirror body 18 made of low expansion glass by a plurality of finger elements 20. The finger elements extend axially from the membrane 14 and are rigidly attached to a cylindrical ring 22 of the mirror. Deflection forces are applied to the mirror by an axial driver screw 24 which acts through a backing plate 25 to bear on a central hub 26 and move a reflective surface portion 28 of the mirror. Thus, an electrical input applied to the piezoelectric elements 10 and 12 causes the screw 24 to move axially and displace the reflective surface portion in the axial direction.
The structure of FIGS. 5A and 5B eliminates errors due to thermal expansion of the piezoelectric elements but causes other thermal errors. The metallic driver body 16 expands more than the low expansion glass of the mirror body 18 and thus introduces thermal effects requiring compensation. The amount of mirror motion required to compensate for this can be as high as 45 microinches (1.143 microns), which must be designed into the device along with the required motion of at least 135 microinches (3.43 microns) for a 28-centimeter gyro. In addition, differential expansion produces asymmetric distortion and accompanying output errors when the driver body 16 is slightly off center relative to the mirror 18 or when the cement holding the finger elements 20 to the cylindrical ring 22 is not applied uniformly.
Other problems with the device of FIGS. 5A and 5B can be traced to the screw contacting the hub of the mirror. The screw applies a preset bias to the mirror 18 and can move laterally relative to the surface of the mirror in use. The screw can even "unload" relative to the mirror in high frequency operation, resulting in a loss of mirror control and possible chipping of the contacted surface.
It is therefore desirable to provide a path length control device having reduced thermal and vibrational effects and low gyro error. It is also desirable to provide a path length control device having a high capacity for compensation and a large operating temperature range.